The purpose of using a lining system is to provide a "impermeable" barrier between contaminants and ground water. Generally, these liners are made of insulating material such as high density polyethylene. For example, Gundle Lining Systems, Inc. of Houston, Tex. has a high density polyethylene liner available in thicknesses of 1 millimeter to 3.5 millimeters.
Synthetic liners, manufactured under stringent quality control standards, are thoroughly tested to be defect free at the time of shipment. However, during installation the liner is exposed to a wide spectrum of hazards such as heavy equipment, cutting tools, welding equipment, animals, and vandalism. Even the most stringent quality control program cannot protect against every hazard. Therefore, a final leak check may be conducted after the liner is installed to locate leaks caused by construction damage. These leak tests are designed not only to determine whether or not a leak exists, that is, to detect the leak, but they also hopefully will succeed in locating the leak so that it may be repaired.
Conventional leak detection and location techniques include smoke, traceable gas, electric surveys, and the like. However, in the case of smoke and traceable gas, since there can be no positive assurance that the indicating media has reached the entire surface under the liner, holes that may cause leaks may go undetected.
Electrical surveys require the liner to be in contact with an electrically conductive media both above and below the liner. This normally means that not only must the liner contain a liquid but also the soil upon which the liner rests must be conductive. The resulting water pressure on top of the liner generally maintains the liner in contact with the subgrade. However, contact with the earth is not always maintained because of irregularities in the subgrade and wrinkles in the liner.
Primary liners in a double liner system cannot be tested in this way because the space between the liners would have to be flooded in order to have electrically conductive media on both sides of the primary liner, and this would cause the primary liner to float. Similarly, side slopes are normally not tested due to the extreme difficulty of maintaining a water volume sufficient to cover the sides as well as the bottom of the liner, and because the resulting depth of the water makes testing difficult.
In known systems, the entire liner surface must be surveyed to detect and locate all defects. After the defects are discovered, since the leaks cannot be repaired under water, it is necessary to drain the liquid from the cover, make repairs, refill and survey again to ensure that all defects have been located and repaired. This process is time consuming and expensive particularly with large lined areas.
In a commercially available apparatus for electrically detecting liner leaks, a potential is induced across the thickness of a liner. If a potential of one polarity is induced on one side of the sheet and a potential of the opposite polarity is induced on the opposite side of the sheet, the resulting electrical field will be affected if there is any conductivity from side to side across the sheet. The effects of the conduction can be sensed to detect the presence of a leak.
Existing system can detect leaks through pin holes as small as 1.0 millimeter in diameter. Even such small holes may cause leaks on the order of a couple of gallons per day with one foot of water pressure. Thus, the presence of even small holes in such liners is of considerable significance. One author has recently indicated that on average there are about 26 holes per ten thousand square meters of liner. Peggs, "Detection and Investigation of Leaks in Geomembrane Liners," Geosynthetics World, Winter 1990. Particularly where hazardous materials are involved, it is extremely important that these holes be located and repaired prior to placing the impoundment in service.
The accuracy of existing methods is limited by the fact that both sides of the liner must be in contact with a conductive medium. In the case of a double liner, the upper or primary liner is not in contact with a conductive medium on both sides. With respect to the lower or secondary liner or in a situation where there is only one liner, a problem arises because a liner may not be in good electrical contact with the earth. Moreover, where the earth is dry or not conductive, the system may not be reliable.
It has been suggested that the liner could be placed in contact with a conductive foil sheet. In U.S. Pat. No. 3,252,155 to Surtees et al., a liner is placed over or even adhesively secured to a metal foil sheet. While that system would appear to overcome some of the problems in the prior art, this technique has not achieved acceptance in the industry. It is likely that any type of exposed metal foil would be severely degraded at the construction site. Moreover, adhesively or mechanically securing the metal foil to the linear would be extremely difficult to achieve, particularly in situ. Even if the foil could be effectively attached, it would be expensive and would raise the possibility of galvanic corrosion.
Thus, a need continues to exist for a liner which may be more easily and economically inspected using an electrical inspection apparatus without the necessity of a liquid medium within the reservoir and without the need for maintaining good electrical contact with conductive natural surroundings outside the liner.
In accordance with the present invention, a method and apparatus is provided to facilitate the detection of holes in liners. In accordance with one aspect of the present invention, a method and apparatus is provided to facilitate to the detection of holes in the plastic sheeting itself.